Activation of the endocannabinoid system through CB1, CB2 and additional receptor subtypes results in the inhibition of a broad range of cancers. The endocannabinoid system was discovered through research focusing on the classical cannabinoid agonist, ?9-tetrahydrocannabinol (?9-THC), and other synthetic cannabinoids. This proposal will focus on the potential treatment of human breast cancer using cannabinoids as selective antitumor agents. We have found that cannabinoid compounds activating CB1, CB2 and additional receptor subtypes can inhibit breast cancer cell proliferation and invasiveness and we have discovered down-stream targets that potentially link cannabinoid receptor stimulation to these effects. Furthermore, our preliminary studies provide evidence that endogenous endocannabinoid tone tonically inhibits metastatic breast cancer cell proliferation and invasiveness through the activation of cannabinoid receptors. Our preliminary data also suggests that cannabinoid compounds possess selective efficacy, having less adverse effects on the normal human cells from which the breast cancers arise. Since toxicity in healthy tissue limits the efficacy of current cancer treatments, discovering the mechanism behind selective efficacy in human tissues is of clinical importance. Cannabinoids can inhibit multiple types of tumor growth in\ vivo, however, this has not been determined for breast cancer. The first aim of this application is to determine if selective CB2 receptor agonists can inhibit metastatic breast cancer cell proliferation and invasiveness as effectively as mixed CB1/CB2 receptor agonists. CB2 agonists do not produce psychotropic side effects, as compared to agonists with CB1 receptor activity, and would potentially be more useful clinically. Next, signal transduction pathways potentially involved in control of breast cancer cell proliferation and invasion by cannabinoids will be evaluated. The next aim will be to evaluate the magnitude of selective efficacy that cannabinoids have for inhibiting breast cancer cell proliferation, migration, and inducing cell death. We will also define the relationship of these effects to alterations in downstream targets. The cannabinoid, cannabidiol, does not interact with CB1 and CB2 receptors, but can inhibit breast cancer cell proliferation and invasiveness. The third aim will be to determine if activation of a cannabinoid receptor subtype and specific signaling pathways are responsible for the effects of cannabidiol. The final aim will be to determine if cannabinoids can inhibit breast cancer metastasis in vivo. Testing the hypotheses outlined in the application may lead to the development of effective inhibitors of breast, and perhaps other, cancers. This research may also elucidate novel mechanisms related to the anticancer activity of cannabinoids, and will serve to develop the career of the candidate in the field of cancer biology.